1. Field of the Invention
The present invention relates to a method of forming in an earthen foundation a modified ground which is used for an underground water cut-off wall, landslide protection wall, or a foundation pile in the earthen foundation, and for reinforcing a soft earthen foundation.
2. Disclosure of the Prior Art
Japanese Patent Early Publication No. 04-299558 discloses a method of forming a pillar in an earthen foundation. The method uses an excavator with an augur shaft having a bit for excavating the earthen foundation, a spiral screw and nozzles for jetting a consolidating fluid. The method comprises the steps of excavating the earthen foundation with the bit to form therein a hole while rotating the augur shaft and jetting the consolidating fluid from the nozzles until the hole reaches to a predetermined depth, and withdrawing the augur shaft from the hole. As a result, the hole is filled with a mixture of the consolidating fluid and the excavated soil. After a solidification of the mixture, the pillar is formed in the earthen foundation. A diameter of the pillar is substantially equal to a diameter of gyration of the spiral screw of the augur shaft. Therefore, the jetting of the consolidating fluid in the excavating step is useful for performing an in-situ stirring and mixing of the excavated soil and the consolidating fluid without varying the diameter of the hole. Of course, it is possible that the prior method is repeated to construct an underground water cut-off wall or landslide protection wall in the earthen foundation.
By the way, as a huge structure is constructed in or on the earthen foundation, a method of forming a thick pillar or long pillar is desired. For example, it would be readily proposed by one of ordinary skill in the art that the thick pillar is formed by the use of an excavator having a thick augur shaft. However, since such an excavator needs a powerful and expensive driving device for rotating the thick augur shaft, it would cause an excess construction cost of the thick pillar as a whole. In addition, since the augur shaft always receives a contacting pressure between the bit and the earthen foundation during the excavating step, there is a problem that the augur shaft can be accidentally advanced into the earthen foundation along an inclined axis with respect to the earthen foundation, so that an inclined hole would be formed in the earthen foundation. Subsequently, in the withdrawing step, the augur shaft is withdrawn away from the inclined hole without modifying a vertical accuracy of the inclined hole. Therefore, in accordance with the prior method, it would be difficult to form a modified ground pillar having an improved vertical accuracy thereof in the earthen foundation.